The present invention generally relates to turbine engines and, more particularly, to a method and system used in electrical control sensors for shaft speed signal frequency change rate tests.
Compressor and load shaft speeds are the primary control parameters of gas turbine engines. Accurate speed measurement is essential for proper engine control. In modern engines with electronic control systems, shaft speeds are typically measured using passive variable reluctance magnetic speed sensors, which sense passing of gear teeth or similar objects. The sensors output an electrical signal to the gas turbine electronic control unit (ECU), with signal frequency proportional to the shaft speed (i.e., passing speed of the gear teeth). The ECU measures the speed by measuring the frequency of the speed pickup signal. The ECU typically conducts reasonableness tests to insure the accuracy of the signal before using it. These may include sensor impedance tests (to check whether the electrical characteristics of the sensor appear normal), and signal frequency range and change rate tests (to check whether the resulting signal characteristics appear normal, within the expected range and not changing at an unreasonable rate).
The conventional signal frequency change rate tests used to detect intermittent or “in-range” failures are unreliable because they either often detect failures that do not truly exist (false alarms) or, in order to avoid generation of false alarms, they miss real failure events. There are four typical failure modes that need to be addressed by signal frequency change rate type tests. First failure mode includes intermittent electrical sensor failures that cause a noisy signal. The other three failure modes include “in-range” failures. Second failure mode includes internal sensor failures which can cause “multiple crossings” or cases where higher than normal speeds are read occasionally. Third failure mode includes damaged gear teeth, shaft runout or excessive speed pickup installation gaps, and can cause “missed teeth” and resultant speed measurement errors. Some conventional controllers even have added sophisticated hardware circuits to detect “missing teeth”. On turbofans, the fourth failure mode is a catastrophic engine failure event called a “blade out”, which causes the controller to perceive speed incorrectly and fuel the engine up.
As can be seen, there is a need for a method and system for implementing signal frequency change rate tests, useable for detection of four intermittent or “in-range” failure modes discussed above, which is more reliable and less complex.